rt2x00: Clean up Kconfig for RT2800 devices.

General clean up of the Kconfig part for RT28XX devices.
Also remove the indications of non functional support for rt27xx/rt28xx/rt30xx
devices, as this is no longer true. They just work fine.
Finally, remove the experimental indications for rt27xx/rt28xx/rt30xx devices
as that is no longer true. Keep the experimental indications for rt33xx/rt35xx
devices, though.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>